The field of the invention is the detection and imaging of objects using acoustic beams.
In the field of medical imaging there are a number of modes in which ultrasound can be used to produce images of objects within a patient. The ultrasound transmitter may be placed on one side of the object and the sound transmitted through the object to the ultrasound receiver placed on the other side ("transmission" mode). With transmission mode methods, an image may be produced in which the brightness of each image pixel is a function of the amplitude of the ultrasound that reaches the receiver ("attenuation mode"), or the brightness of each pixel is a function of the time required for the sound to reach the receiver ("time-of-flight" or "speed of sound" mode). In the alternative, the receiver may be positioned on the same side of the object as the transmitter and an image may be produced in which the brightness of each pixel is a function of the amplitude of the ultrasound reflected from the object back to the receiver ("reflection", "backscatter" or "echo" mode). In another mode of operation ("Doppler" mode) the movement of the object is detected and imaged by measuring the phase of the ultrasound reflected from the object back to the receiver.
In all of these medical imaging applications ultrasonic waves are transmitted and ultrasonic waves are received. The higher sonic frequencies enable precise beams to be formed in both the transmit and receive modes. Ultrasonic transducers for medical applications are constructed from one or more piezoelectric elements sandwiched between a pair of electrodes. Such piezoelectric elements are typically constructed of lead zirconate titanate (PZT), polyvinylidene diflouride (PVDF), or PZT ceramic/polymer composite. The electrodes are connected to a voltage source, and when a voltage waveform is applied, the piezoelectric elements change in size at a frequency corresponding to that of the applied voltage. When a voltage waveform is applied, the piezoelectric elements emits an ultrasonic wave into the media to which it is coupled at the frequencies contained in the excitation waveform. Conversely, when an ultrasonic wave strikes the piezoelectric element, the element produces a corresponding voltage across its electrodes. A number of such ultrasonic transducer constructions are disclosed in U.S. Pat. Nos. 4,217,684; 4,425,525; 4,441,503; 4,470,305 and 4,569,231.
When used for ultrasonic imaging, the transducer typically has a number of piezoelectric elements arranged in an array and driven with separate voltages (apodizing). By controlling the time delay (or phase) and amplitude of the applied voltages, the ultrasonic waves produced by the piezoelectric elements (transmission mode) combine to produce a net ultrasonic wave that travels along a preferred beam direction and is focused at a selected point along the beam. By controlling the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the subject. A number of such ultrasonic imaging systems are described in U.S. Pat. Nos. 4,155,258; 4,155,260; 4,154,113; 4,155,259; 4,180,790; 4,470,303; 4,662,223; 4,669,314; 4,809,184; 5,081,995 and 5,492,121.
The acoustic radiation force exerted by an acoustic wave on an object in its path is a universal phenomenon common to all forms of radiated energy. When a beam of light is absorbed or reflected by a surface, a small force is exerted on that surface. The same is true for electromagnetic waves, transverse waves on an elastic string, and surface waves on a liquid. This force is produced by a "radiated pressure" and a complete disclosure of this phenomenon is set forth by G. R. Torr, "The Acoustic Radiation Force", Am. J. Phys. 52(5), May 1984.
The measurement of radiation force exerted by sound waves has become important in recent years to determine the power outputs of medical imaging ultrasonic transducers, Beissner, K., "Measurement Techniques In Ultrasonic Exposimetry," eds. M. C. Ziskin and P. A. Lewin, CRC Press, Boca Raton, 1993. The transducer is submerged in a tank of water and the ultrasonic beam is directed towards an absorbing or reflecting target in the tank. An absorbing target may be realized by a slab of natural rubber, or a reflecting target by an air-backed thin metal plate. If the ultrasonic beam is directed horizontally, the force can be determined by suspending the target as a pendulum and measuring its deflection. The measurements are made in water because the characteristic acoustic impedances of water and human soft tissue are similar, thus the measured ultrasonic beam power is virtually equal to the power radiated by the transducer into the human body provided that the effect of tissue loss has been accounted for.
It is generally accepted that the radiation force F exerted on a totally absorbing target by an ultrasonic beam of power P is given by the equation EQU F=P/c,
where c is the speed of sound in the medium surrounding the target. For normal incidence on a plane reflecting surface the radiation force has twice this value. The speed of sound in water is 1500 m/s, thus the radiation force on an absorbing target in water is about 6.67.times.10.sup.-4 newtons/watt.
This sonic radiation force has found application in medicine in the field of extracorporeal shock wave lithotripsy. By applying a set of powerful acoustic shock waves at the surface of the patient such that their energies focus on a target inside the patient, objects such as renal or gall-stones can be fragmented. Such lithotripsy systems are described, for example, in Goldstein, A., "Sources of Ultrasonic Exposure," Ultrasonic Exposimetry, eds. M. C. Ziskin and P. A. Lewin, CRC Press, Boca Raton, 1993.
Another application which employs an ultrasonic radiation force produced by a transducer is disclosed by Sugimoto et al, "Tissue Hardness Measurement Using The Radiation Force Of Focused Ultrasound", IEEE Ultrasonics Symposium, pp. 1377-80, 1990. In this experiment, a pulse of focused ultrasonic radiation is applied to deform the object which is positioned at the focal point of the transducer. The deformation is measured using a separate pulse-echo ultrasonic system and the hardness of the deformed object is measured. Measurements are made based on the rate of object deformation as the acoustic force is continuously applied, or by the rate of relaxation of the deformation after the force is removed.
A similar system is disclosed by T. Sato, et al. "Imaging of Acoustical Nonlinear Parameters and Its Medical and Industrial Applications: A Viewpoint as Generalized Percussion", Acoustical Imaging, Vol. 20, pg 9-18, published in 1993 by Plenum Press. In this system a lower frequency wave (350 kHz) is produced to act as a percussion force, and an ultrasonic wave (5 MHz) is used in a pulse echo mode to produce an image of the subject. The percussion force is said to perturb second order nonlinear interactions in tissues, which reveal more structural information than the conventional ultrasonic pulse/echo system alone.